1. Field of the Invention
The present invention relates to a starting circuit of a starting device for a high-pressure discharge lamp including an auxiliary light source, the starting circuit being capable of reliably starting the high-pressure discharge lamp including the auxiliary light source.
2. Description of the Related Art
A high-pressure discharge lamp is mainly provided for a lighting system which is used for a liquid crystal projector and an optical device such as an exposure device. In recent years, an auxiliary light source is arranged in a high-pressure discharge lamp in response to demand for reduction in initial start (cold start) time as well as in restart (hot start) time and in response to demand for lowering the voltage required for starting (e.g., Patent document 1: Japanese Laid-Open Patent Publication No. 2003-203605).
As shown in FIG. 9, an auxiliary light source 3 arranged in a lighting system 1 described in Patent document 1 includes a discharge chamber 7 having discharge space 7a enclosing therein material M2 which emits ultraviolet rays when excited by discharge, and a starting electrode 8 that is arranged so as to be opposite via the discharge chamber 7 to one of metal foils 6b which is embedded in one of sealing portions 5b of a high-pressure discharge lamp 2. A conductive wire 9 is electrically connected to the starting electrode 8 so as to apply high-frequency high voltage between the one of the metal foils 6b and the starting electrode 8. Such a configuration that has electrodes arranged outside the discharge space 7a is called “electrodeless type”.
In order to start the high-pressure discharge lamp 2 of the lighting system 1, high-frequency high voltage is applied between the one of the metal foils 6b and the starting electrode 8. Dielectric barrier discharge is then generated between the one of the metal foils 6b and the starting electrode 8 via the discharge space 7a of the discharge chamber 7. The material M2 enclosed in the discharge space 7a is excited by the dielectric barrier discharge, thereby emitting ultraviolet rays UV. The ultraviolet rays UV irradiate light-emitting material M1 enclosed in a light-emitting portion 5a in the high-pressure discharge lamp 2, whereby the light-emitting material M1 is ionized. Consequently, discharge between main discharge electrodes 6a is accelerated, whereby the high-pressure discharge lamp 2 is started by applying lower voltage.
In order to start the auxiliary light source of the above-described “electrodeless type”, high-frequency high voltage needs to be applied to the auxiliary light source so as to produce capacitive coupling and then to cause excitation. Accordingly, various types of starting devices for the high-pressure discharge lamp, which are capable of generating high-frequency high voltage, have been developed (e.g., Patent document 2: Japanese Laid-Open Patent Publication No. 2007-109510).
In a starting device 100 for a high-pressure discharge lamp disclosed in Patent document 2, as shown in FIG. 10, a voltage generation circuit 104 and an inverter 106 are connected to a high-pressure discharge lamp 102 via an output line 108 and the other output line 110 so as to supply stabilized electric power to the high-pressure discharge lamp 102. The output line 108 is connected to a secondary winding 114 of a transformer 112. A primary winding 116 of the transformer 112 and a capacitor 118 are connected in parallel with each other to constitute a parallel resonant circuit. Voltage having predetermined frequency is applied from periodical voltage applying means 120 to the primary winding 116 and the capacitor 118. That is, a starting circuit 122 for the high-pressure discharge lamp 102 is composed of the transformer 112, the capacitor 118, and the periodical voltage applying means 120.
To start the high-pressure discharge lamp 102, periodical voltage is applied from the periodical voltage applying means 120 to the primary winding 116 and the capacitor 118 (i.e., the parallel resonant circuit). When frequency of the voltage applied from the periodical voltage applying means 120 corresponds to fundamental resonant frequency or high-order resonant frequency of the parallel resonant circuit, resonant current flows through the parallel resonant circuit, and high voltage is generated in the primary winding 116. With the high voltage generated in the primary winding 116, higher voltage is generated in the secondary winding 114, the higher voltage being boosted in accordance with a turns ratio (a ratio of the number of turns of wire in the primary winding 116 to the number of turns of wire in the secondary winding 114).
The higher voltage generated in the secondary winding 114 is superimposed with output voltage from the voltage generation circuit 104 and the inverter 106, and the resultant voltage is applied to the high-pressure discharge lamp 102, whereby high-frequency high voltage can be applied to the high-pressure discharge lamp 102.
However, the starting device 100 for the high-pressure discharge lamp disclosed in Patent document 2 has the following problems. That is, in the starting device 100 for the high-pressure discharge lamp, the secondary winding 114 of the transformer 112 is placed in the output line 108, and high rated current is fed to the high-pressure discharge lamp 102 through the secondary winding 114 when light is emitted steadily. Therefore, there has been required a large-size, expensive transformer 112 which includes a secondary winding 114 capable of accommodating high rated current. As a result, the starting device 100 for the high-pressure discharge lamp needs to be upsized and thus becomes expensive.